Information Technology Reference
In-Depth Information
DUAL selects a best path and a second-best path to reach a destination. The best path se-
lected by DUAL is the
successor
, and the second-best path (if available) is the
fe a s i b l e
successor
. The feasible distance is the lowest calculated metric of a path to reach the des-
tination. The topology table in Example 10-6 shows the feasible distance. The example
also shows two paths (Ethernet 0 and Ethernet 1) to reach 172.16.4.0/30. Because the
paths have different metrics, DUAL chooses only one successor.
Example 10-6
Fea s ibl e D ista nc e a s Sh ow n in th e EIGRP Top ol og y Tabl e
Router8#
show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(172.16.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 172.16.4.0/30, 1 successors, FD is 2195456
via 172.16.1.1 (2195456/2169856), Ethernet0
via 172.16.5.1 (2376193/2348271), Ethernet1
P 172.16.1.0/24, 1 successors, FD is 281600
via Connected, Ethernet0
The route entries in Example 10-6 are marked with a P for the passive state. A destination
is in passive state when the router is not performing any recomputations for the entry. If
the successor goes down and the route entry has feasible successors, the router does not
need to perform any recomputations and does not go into active state.
DUAL places the route entry for a destination into active state if the successor goes down
and there are no feasible successors. EIGRP routers send query packets to neighboring
routers to find a feasible successor to the destination. A neighboring router can send a re-
ply packet that indicates it has a feasible successor or a query packet. The query packet in-
dicates that the neighboring router does not have a feasible successor and will participate
in the recomputation. A route does not return to passive state until it has received a reply
packet from each neighboring router. If the router does not receive all the replies before
the “active-time” timer expires, DUAL declares the route as stuck in active (SIA). The de-
fault active timer is 3 minutes.
EIGRP Timers
EIGRP sets updates only when necessary and sends them only to neighboring routers.
There is no periodic update timer.
EIGRP uses hello packets to learn of neighboring routers. On high-speed networks, the
default hello packet interval is 5 seconds. On multipoint networks with link speeds of T1
and slower, hello packets are unicast every 60 seconds.
The holdtime to maintain a neighbor adjacency is 3 times the hello time: 15
router does not receive a hello within the holdtime, it removes the neighbor
